747 research outputs found
Rotation among solar-type stars
The net result is that FRESIP will produce high quality rotational data for a large (and essentially volume-limited) sample of solar-type stars. Moreover, the observed slight changes in rotation period will indicate the nature and degree of differential rotation on those stars. Both parameters are central to our understanding of the evolution of the Sun
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Photometric modeling of a cometary nucleus: taking Hapke modeling to the limit
Hubble Space Telescope Goddard High Resolution Spectrograph (GHRS) Instrument Handbook (for the Post-COSTAR Observatory), version 5.0
This Handbook exists as a basic reference manual for the Goddard High Resolution Spectrograph (GHRS), and describes its properties and operation
White Dwarf Cosmochronology in the Solar Neighborhood
The study of the stellar formation history in the solar neighborhood is a
powerful technique to recover information about the early stages and evolution
of the Milky Way. We present a new method which consists of directly probing
the formation history from the nearby stellar remnants. We rely on the volume
complete sample of white dwarfs within 20 pc, where accurate cooling ages and
masses have been determined. The well characterized initial-final mass relation
is employed in order to recover the initial masses (1 < M/Msun < 8) and total
ages for the local degenerate sample. We correct for moderate biases that are
necessary to transform our results to a global stellar formation rate, which
can be compared to similar studies based on the properties of main-sequence
stars in the solar neighborhood. Our method provides precise formation rates
for all ages except in very recent times, and the results suggest an enhanced
formation rate for the solar neighborhood in the last 5 Gyr compared to the
range 5 < Age (Gyr) < 10. Furthermore, the observed total age of ~10 Gyr for
the oldest white dwarfs in the local sample is consistent with the early
seminal studies that have determined the age of the Galactic disk from stellar
remnants. The main shortcoming of our study is the small size of the local
white dwarf sample. However, the presented technique can be applied to larger
samples in the future.Comment: 25 pages, 10 figures, accepted for publication in the Astrophysical
Journa
Ages of young stars
Determining the sequence of events in the formation of stars and planetary
systems and their time-scales is essential for understanding those processes,
yet establishing ages is fundamentally difficult because we lack direct
indicators. In this review we discuss the age challenge for young stars,
specifically those less than ~100 Myr old. Most age determination methods that
we discuss are primarily applicable to groups of stars but can be used to
estimate the age of individual objects. A reliable age scale is established
above 20 Myr from measurement of the Lithium Depletion Boundary (LDB) in young
clusters, and consistency is shown between these ages and those from the upper
main sequence and the main sequence turn-off -- if modest core convection and
rotation is included in the models of higher-mass stars. Other available
methods for age estimation include the kinematics of young groups, placing
stars in Hertzsprung-Russell diagrams, pulsations and seismology, surface
gravity measurement, rotation and activity, and lithium abundance. We review
each of these methods and present known strengths and weaknesses. Below ~20
Myr, both model-dependent and observational uncertainties grow, the situation
is confused by the possibility of age spreads, and no reliable absolute ages
yet exist. The lack of absolute age calibration below 20 Myr should be borne in
mind when considering the lifetimes of protostellar phases and circumstellar
material.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
Lithium abundances from the 6104A line in cool Pleiades stars
Lithium abundances determined by spectral synthesis from both the 6708A
resonance line and the 6104 subordinate line are reported for 11 late-type
Pleiades stars, including spectra previously analysed by Russell (1996). We
report a 0.7 dex scatter in the abundances from 6708A, and a scatter at least
as large from the 6104A line. We find a reasonable correllation between the
6104A and 6708A Li abundances, although four stars have 6104A-determined
abundances which are significantly larger than the 6708-determined values, by
up to 0.5 dex, suggesting problems with the homogeneous, one-dimensional
atmospheres being used. We show that these discrepancies can be explained,
although probably not uniquely, by the presence of star spots with plausible
coverage fractions. The addition of spots does not significantly reduce the
apparent scatter in Li abundances, leaving open the possibility that at least
some of the spread is caused by real star-to-star differences in pre-main-
sequence Li depletion.Comment: 13 pages, 7 figures; Accepted by A&A 17/05/0
Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association
The associations and moving groups of young stars are excellent laboratories
for investigating stellar formation in the solar neighborhood. Previous results
have confirmed that a non-negligible fraction of old main-sequence stars is
present in the lists of possible members of young stellar kinematic groups. A
detailed study of the properties of these samples is needed to separate the
young stars from old main-sequence stars with similar space motion, and
identify the origin of these structures. We used stars possible members of the
young (~ 10 - 650 Myr) moving groups from the literature. To determine the age
of the stars, we used several suitable age indicators for young main sequence
stars, i.e., X-ray fluxes and other photometric data. We also used
spectroscopic data, in particular the equivalent width of the lithium line Li I
and Halpha, to constrain the range of ages of the stars. By combining
photometric and spectroscopic data, we were able to separate the young stars
(10 - 650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that
the Local Association is contaminated by old field stars at the level of ~30%.
This value must be considered as the contamination for our particular sample,
and not of the entire Local Association. For other young moving groups, it is
more difficult to estimate the fraction of old stars among possible members.
However, the level of X-ray emission can, at least, help to separate two age
populations: stars with <200 Myr and stars older than this. Our results are
consistent with a scenario in which the moving groups contain both groups of
young stars formed in a recent star-formation episode and old field stars with
similar space motion. Only by combining X-ray and optical spectroscopic data is
it possible to distinguish between these two age populations.Comment: 7 pages, 7 figures. Accepted for publication in A&
Chromospheric activity, lithium and radial velocities of single late-type stars possible members of young moving groups
We present here high resolution echelle spectra taken during three observing
runs of 14 single late-type stars identified in our previous studies (Montes et
al. 2001b, hereafter Paper I) as possible members of different young stellar
kinematic groups (Local Association (20 - 150 Myr), Ursa Major group (300 Myr),
Hyades supercluster (600 Myr), and IC 2391 supercluster (35 Myr)). Radial
velocities have been determined by cross correlation with radial velocity
standard stars and used together with precise measurements of proper motions
and parallaxes taken from Hipparcos and Tycho-2 Catalogues, to calculate
Galactic space motions (U, V, W) and to apply Eggen's kinematic criteria. The
chromospheric activity level of these stars have been analysed using the
information provided for several optical spectroscopic features (from the Ca II
H & K to Ca II IRT lines) that are formed at different heights in the
chromosphere. The Li I 6707.8 AA line equivalent width (EW) has been determined
and compared in the EW(Li I) versus spectral type diagram with the EW(Li I) of
stars members of well known young open clusters of different ages, in order to
obtain an age estimation. All these data allow us to analyse in more detail the
membership of these stars in the different young stellar kinematic groups.
Using both kinematic and spectroscopic criteria we have confirmed PW And, V368
Cep, V383 Lac, EP Eri, DX Leo, HD 77407, and EK Dra as members of the Local
Association and V834 Tau, pi^{1} UMa, and GJ 503.2 as members of the Ursa Major
group. A clear rotation-activity dependence has been found in these stars.Comment: Latex file with 19 pages, 7 figures tar'ed gzip'ed. Full postscript
(text, figures and tables) available at
http://www.ucm.es/info/Astrof/p_skg_stars_I_fv.ps.gz Accepted for publication
in: Astronomy & Astrophysics (A&A
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